An overcurrent condition display is used mainly as a means for locating promptly those spots on a distribution cable where overcurrent conditions have occurred. The display is required usually to remain in a display state for 3 to 7 hours necessary for inspections and to be restored to a normal state after the lapse of a predetermined time.
One known device of this kind is suspended directly from a distribution cable and set in operation by an overcurrent sensed by a current transformer. However, the device of this kind is actuated mechanically by the direct electromagnetic force of the sensed overcurrent and hence, for practical purposes, the mechanical operation of the display must be more speedy than that of overcurrent relays at a substation. In effect, a fault current is usually of the order of magnitude of 600 to 1000 amperes and lasts only almost in an instant. With the electric power of such character, it is extremely difficult to actuate a display instantly, to maintain the display state for a prescribed time and yet to restore the display automatically to the original normal state. In particular, it is really difficult to establish a prolonged time limit of the order of several hours with a simple and maintainance-free mechanism.